// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package gzip0 implements writing of uncompressed (compression level 0) gzip files.
// The data in the files is uncompressed but still framed and checksummed,
// protecting against undetected corruption or truncation.
package gzip0

import "io"

// A Writer is an io.WriteCloser.
// Writes to a Writer are written to w in uncompressed gzip format.
type Writer struct {
	started bool
	w       io.Writer
	buf     [32]byte
	size    int64
	crc32   uint32
	err     error
}

// NewWriter returns a new Writer.
// Writes to the returned Writer are written to w in uncompressed gzip format.
//
// It is the caller's responsibility to call Close on the WriteCloser when done.
// Writes may be buffered and not flushed until Close.
func NewWriter(w io.Writer) *Writer {
	return &Writer{w: w}
}

// buffer returns a temporary slice holding the given data,
// to use as the argument of a w.w.Write call.
func (w *Writer) buffer(data ...byte) []byte {
	copy(w.buf[:len(data)], data)
	return w.buf[:len(data)]
}

// start writes the gzip header if it hasn't been written yet.
func (w *Writer) start() error {
	if w.err != nil {
		return w.err
	}
	if w.started {
		return nil
	}
	w.started = true
	_, w.err = w.w.Write(w.buffer(0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xff))
	return w.err
}

// Write writes a compressed form of p to the underlying io.Writer. The
// compressed bytes are not necessarily flushed until the Writer is closed.
func (w *Writer) Write(b []byte) (int, error) {
	if err := w.start(); err != nil {
		return 0, err
	}
	total := len(b)
	for len(b) > 0 {
		n := len(b)
		if n > 0xffff {
			n = 0xffff
		}
		_, w.err = w.w.Write(w.buffer(0x00, byte(n), byte(n>>8), ^byte(n), ^byte(n>>8)))
		if w.err != nil {
			return total - len(b), w.err
		}
		n, w.err = w.w.Write(b[:n])
		w.size += int64(n)
		w.crc32 = updateCRC32(w.crc32, b[:n])
		b = b[n:]
		if w.err != nil {
			return total - len(b), w.err
		}
	}
	return total, nil
}

// Close closes the Writer, flushing any unwritten data to the underlying
// io.Writer, but does not close the underlying io.Writer.
func (w *Writer) Close() error {
	if err := w.start(); err != nil {
		return err
	}
	_, w.err = w.w.Write(w.buffer(0x01, 0x00, 0x00, 0xff, 0xff, byte(w.crc32), byte(w.crc32>>8), byte(w.crc32>>16), byte(w.crc32>>24), byte(w.size), byte(w.size>>8), byte(w.size>>16), byte(w.size>>24)))
	if w.err != nil {
		return w.err
	}
	return nil
}

func updateCRC32(crc uint32, p []byte) uint32 {
	crc = ^crc
	for _, v := range p {
		crc = crctab[byte(crc)^v] ^ (crc >> 8)
	}
	return ^crc
}

var crctab = [256]uint32{
	/*
	   package main

	   import "fmt"

	   func main() {
	   	const poly = 0xedb88320
	   	for i := 0; i < 256; i++ {
	   		crc := uint32(i)
	   		for j := 0; j < 8; j++ {
	   			if crc&1 == 1 {
	   				crc = (crc >> 1) ^ poly
	   			} else {
	   				crc >>= 1
	   			}
	   		}
	   		fmt.Printf("%#08x, ", crc)
	   		if i%8 == 7 {
	   			fmt.Printf("\n")
	   		}
	   	}
	   }
	*/
	0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
	0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
	0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
	0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
	0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
	0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
	0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
	0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
	0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
	0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
	0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
	0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
	0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
	0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
	0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
	0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
	0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
	0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
	0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
	0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
	0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
	0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
	0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236, 0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
	0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
	0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
	0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
	0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
	0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
	0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
	0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
	0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
	0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d,
}
